1. Field of the Invention
This invention relates to a novel cutting and lubricating composition for use with an apparatus for cutting workpieces of hard and brittle material such as semiconductor ingots of silicon, germanium, gallium arsenide, glass or other brittle materials, such as granite block, into a multiplicity of thin sheets, slices or wafers.
2. Description of the Prior Art
The cutting apparatus usually comprises a row of fine wires arranged parallel to each other and at a fixed pitch. A workpiece is pressed against these fine wires having diameters in the order of 0.2 millimeters running in parallel with one another in the same direction, while an abrasive fluid is supplied between the workpiece and the wires, thereby slicing the workpiece into wafers by a grinding action. Thus, the abrasive particles carried by the liquid are transferred to the cutting sections of the wires to produce a splitting or cutting effect. The above described splitting units or machines are described in U.S. Pat. Nos. 3,478,732, 3,525,324, 5,269,275 and 5,270,271, which are incorporated by reference.
The cutting apparatus may also comprise a series of wires inter-looped or entwined together in a braided loop configuration. This configuration can be used for the cutting of granite block or silicon ingots. The workpiece is pressed against the braided wire and the cutting process is augmented by the abrasive particles as described above.
Achieving an optimum cutting quality depends on a combination of parameters, i.e., the quality of the abrasive fluid and the force with which the workpiece is pressed against the set of cutting wires.
Effort is now being directed to optimizing the cutting quality obtained under mass production conditions. By cutting quality is meant exact planarity of the surfaces without taper and thickness variation to yield products suitable for semiconductor devices and solar cells. Mass production considerations, for example, the rate of wear of the wire, the effectiveness recovery and recycling of the cutting and lubricating fluids are also important. U.S. Pat. No. 5,099,820 issued to Stricot discloses an abrasive liquid as a suspension of particles of silicon carbide in water or oil. However, these prior art suspensions are not stable and do not provide uniform lubrication to the wires. Furthermore, these compositions require vigorous agitation to maintain uniform suspension of the particles, and the suspension settles out quickly under stagnant conditions.
There exists a need for a novel lubricating composition for cutting machines that has excellent lubricity and heat transfer properties. Thus, there also exits a need for a novel cutting and lubricating composition which provides a uniform supply of abrasive material so that the workpiece is more efficiently cut by the abrasive grit in the composition. Further, the composition should have excellent lubricity and heat transfer properties to remove the frictional heat generated at the cutting site thereby increasing working life of the wire and avoiding downtime. Finally, the composition should provide a stable suspension of abrasive particles. However, if stored for a long period and should separation occur, only a gentle agitation Would be required to restore the suspension.